The logic, according to the Journal, was to increase AmEx's profits without making their clients aware.

Sources said the company chose smaller clients because they were less likely to watch exchange rates closely and notice a difference.

American Express pushed back, and said their approach to foreign exchange is "fair and transparent."

The foreign exchange business at American Express reportedly spent more than a decade deliberately hiking its exchange rates for small clients it thought wouldn't notice the difference, current and former employees have told The Wall Street Journal.

According to the Journal, the company's sales team would gain clients by offering them low rates, then later increase them without explicitly saying so. This meant more money for American Express.

Sources said the department increased conversion fees without telling its customers as part of a practice that was widespread since at least 2004 and continued until earlier this year.

According to the Journal, they stopped when allegations of similar practices at Wells Fargo became public.

American Express responded to the Journal by saying that its rates are "fair and transparent." However, they did not specifically deny increasing rates without telling people.

Spokeswoman Marina Norville told Business Insider: "We have training, control and compliance oversight and believe that our transactions are completed and reported in a fair and transparent manner at the rates which the client has authorized."

"Our offered foreign exchange rates are competitive with the current marketplace," she said.

The current and former employees said that they would tell potential clients that they could beat the price they were currently paying to convert and send money abroad. They did not inform these clients that the margin was subject to increase without notice. The salespeople would later increase the margin without telling the customer.

In order to notice the increase, a customer would have to log in to their accounts and compare American Express' rate with the market exchange rate at the time.

Current and former employees and emails reviewed by The Wall Street Journal showed that when customers did notice a change and asked about it, salespeople sometimes would blame a glitch or other technicality and lower the margin for that customer.